An example of such a sensor is a differential confocal sensor, a sensor based on the astigmatic focus detection method, a sensor based on the critical angle detecting method and other sensors of different types.
Such a sensor may be included in a measuring apparatus for measuring for example large rotationally non-symmetrical free form and aspherical optical elements with increasingly tight specifications regarding the shape. From the scientific world and the industry there is an increasing demand for such optical elements and new technologies allow the manufacture of these elements. In the manufacturing process of optical elements, measurement of these elements plays an essential role as a major source of feedback in and between process steps as well as for evaluation of the finished optical element. Thus there is an ever-increasing demand for a high precision contact less, i.e. optical, measuring apparatuses, which shows substantially better resolution and accuracy than conventional apparatuses. An essential part of such an apparatus is the non-contact probe, or optical sensor, which measures the distance to the surface of an object, for example a free form surface. A free-form surface is understood to mean a non-rotationally smooth surface showing an inclination angle with respect to the optical axis of the sensor that varies uni-directionally from 0 degrees to 5 degrees.
In a measuring apparatus under development the optical element to be measured will be arranged on an air-bearing spindle, which rotates, for example at about 1 revolution per second. The optical sensor will be positioned perpendicularly to the rotationally symmetric best-fit contour of the surface under test (hereinafter SUT) by means of an air-bearing sensor-driving R, Z, PSI-mechanism. This mechanism allows movement of the sensor in the vertical (Z) direction, in a horizontal radial (R) direction and rotation of the sensor axis about an angle PSI in a vertical plane such that this axis is at any time substantially perpendicular to the momentarily measured area of the SUT. The measuring apparatus may measure the SUT for example track-wise. When the sensor is measuring a track of the SUT, the R-, Z-, and PSI positions will be stationary. After a first track has been measured, the sensor is positioned on a second track, which is spaced from the first track for example at about 0.5 to 2.2 mm. This is repeated until the complete surface is measured. The apparatus is, for example intended for measuring smoothly curved surfaces for which a track spacing of the order of 2 mm gives sufficient good results.
The envisaged measuring apparatus should satisfy the following requirements:                it should allow universal measurement of free-form surface shapes;        it should use non-contact type measurement;        it should allow measurement of large optical elements (with a diameter of up to 500 mm);        the measurement uncertainty (2 sigma), which will be explained later, should be smaller than 30 nm, and        the apparatus should be fast so that measurement of a surface can be performed within a short time interval.        
An optical sensor for a measuring apparatus that satisfies these requirements should have the following qualities:                contact less measurement;        measurement uncertainty (2 sigma) of at most 10 nm for surfaces perpendicular to the sensor axis, i.e. at 0 degrees inclination, and at most 35 nm for surfaces having inclinations up to 5 degrees;        measurement range up to 5 mm;        resolution down to 1 nm, and        absolute measurement.        
It has appeared that conventional optical measuring methods and apparatuses do not allow measurement of free-form smooth surfaces with the required accuracy, defined herein above.
U.S. Pat. No. 5,424,834 discloses an optical displacement sensor for measurement of shape and coarseness of an object surface, which sensor comprises, in addition to conventional means for measuring the distance to the object surface, a transverse position detection system for measuring transverse displacement of the reflected beam spot. The distance measuring means may be a differential confocal sensor or a so-called astigmatic sensor and the means for measuring transverse displacement may be a position-sensitive detector (PSD).
The PSD in the sensor of U.S. Pat. No. 5,424,834 measures only the transverse displacement of the beam spot relative to the optical axis of the distance measuring system and the measured values are used to control the direction of the reflected measuring beam such that it is always coaxial with the said optical axis. This active control is performed by an actuated mirror, which is called a beam polariscope. This puts a limitation to the measuring rate. Even with the fastest actuator for such a correction mirror a minimum setting time will be needed for correction of the local inclination of the measured surface, so that the correction is delayed. For an optically perfect correction the mirror should be shifted or translated, because it is arranged in a collimated beam, which means that correction is possible only in one direction, or degree of freedom. To correct for inclination, two actuated mirrors would be needed, which would render the system even more complex and slower. A more attractive system, because it is simpler and faster, seems to be a system that uses two tilting mirrors or one mirror than can be tilted in two mutual perpendicular directions. However such a system will show more aberrations and will cause a less accurate performance of the distance measuring part of the sensor. It is also suggested in U.S. Pat. No. 5,424,834 to use, instead of mechanically movable mirrors, a much faster Electro-optical element, but this is a very expensive and complicated solution. An Electro-optical element requires a high voltage and its effect is small so that it would be difficult, if not impossible, to realize the required shift. Moreover, two Electro-optical elements would be needed and the optical path length of the sensor system would increase substantially.
DE 4211875 describes a distance measuring system wherein tilt of the object surface is detected. The measurement beam is split. The split off beam is directed to an additional detector in a way, so that a different dependence on tilt angle occurs in the signal from the additional detector. This document mentions the possibility of on-line tilt correction, using calibration data. This document does not take optical system deficiencies of the sensor into account.
U.S. Pat. No. 4,660,970 and JP 61-140809 likewise disclose object distance sensors. JP 61-140809 provides for a tilt angel measurement.
In view of the requirement that for a non tilted surface area the 2 sigma measurement uncertainty of the distance sensor should be 10 nm or less for a non-tilted surface and 35 nm or less for a surface tilt of 5 degrees, the measurement range for distance measurement is limited to some microns. The 2 sigma measurement uncertainty is understood to mean that there is a chance of 95% that the actual distance is within 35 nm (2 sigma for a surface tilt of 5 degrees) of the measured value. For example an increase to 5 mm is desirable.